The present invention relates to an electrophotographic copying machine capable of quenching unnecessary charges on a non-image area of a photoconductor prior to development of an image thereon.
In the conventional electrophotographic copying machine, the photoconductor is charged uniformly and exposed to a light image and the light image is then developed. At this time, since a non-image area on the photoconductor is not exposed completely, some charges remain on the non-image area, and when the non-image area passes through a development section, toner is attracted to the non image area, which causes wasting of toner and a trouble of the necessity of cleaning the toner deposited on the photoconductor.
Therefore, conventionally, the following quenching systems are proposed. In the first quenching system, between an exposure section and a development section, there is disposed a quenching lamp, whose actuation is controlled in accordance with the timing of the movement of the photoconductor or the timing of exposure, whereby unnecessary charges in the areas before and behind an image area are quenched. However, the unnecessary charges existing in the direction normal to the removement of the photoconductor cannot be quenched.
Therefore, by disposing a plurality of lamps spaced in accordance with the size of an original or the size of a transfer sheet in the direction normal to the movement of the photoconductor, the unnecessary charges are removed. This apparatus is continuously in operation during the copying process. Another known quenching apparatus comprises plural light receiving elements extending in the direction normal to a movement of the photoconductor, which are disposed in the path of light rays which are projected to the exposure section, and the same number of light emitting element as that of the light receiving elements, which are disposed so as to face the surface of the photoconductor. This apparatus is to detect an unexposed area on the photoconductor by the light receiving elements, whereby the quenching lamp is controlled. However, this apparatus is complicated in its mechanism and high in cost.
The second conventional quenching system is comparatively simple in mechanism and works effectively. However, it has the following shortcomings. Namely, in the ordinary electrophotographic copying machine, there is provided a pressure plate having an irregular reflection surface which is white in color on a side thereof facing a contact glass of the electrophotographic copying machine. In order to obtain copies from an original which is smaller in size than a maximum copy size that can be obtained by the copying machine, light from the area outside the original is reflected from the white surface of the pressure plate and reaches the surface of the photoconductor. Therefore, the unnecessary charges in the area outside the image area on the photoconductor can be quenched to some extent without a particular quenching apparatus. However, when copying is performed from a thick original document, the pressure plate is held open or away from the contact glass by the thick original document, so that a sufficient amount of light is not reflected from the pressure plate and accordingly a sufficient amount of light does not reach the photoconductor. Furthermore, there is a risk that the unnecessary charges in the non-image area cannot be removed since copying is performed with the pressure plate opened. Therefore, the unnecessary charges on the photoconductor are removed prior to development by a quenching apparatus. The unnecessary charges in the non-image area can be surely removed by the quenching apparatus. However, in case light reflected from the closed pressure plate reaches the photoconductor, the non-image area is illuminated two times by quenching the charges in the non-image area, although there is no problem when the pressure plate is opened.
Since light fatigue of the photoconductor causes increase of the residual potential on the photoconductor, it is not preferable that the same portion of the photoconductor is exposed two times for such quenching of the unnecessary charges. Particularly in case copies are made from a large original document after repeated copying from a small original document, the surface potential in the preceding image area and that in the preceding non-image area become different, which causes an uneven image density in making copies from the large original.